That one turns on the gas
In the shower scene Sean Connery jokingly chides a fellow prisoner:
"Don't touch that one, it turns on the gas", - all laugh.
At the time (1942) there was still no official acceptance that concentration camp victims were being systematically gassed with Zyklon B whilst ostensibly being given a shower.
A historical innacuracy but there weren't many in this film which was very authentic.
It was a good example of gallows humour though, - witty but not politically correct. Then again, there was no such concept as "political correctness" in 1942, or indeed 1965, when the film was made.
P.S. In the extras section of the Region 1 DVD, a calendar of 1942 is shown, so presumably that was the year depicted in The Hill.